r~
6~ 1976
"Imprc)velllel1t;s in and re:Latirlg to cili:arge collp:lecl cl~vices"
Th:is :invention relates to charge coupled
dcv:ices for conver-tlng an eloctrolnaglletic radiatlon
pattern in a certain wavelength range, part:icul~rly but
' not exclusively an infra-red radiation pat-tern, into
S electrical signals.
Charge coupled devices for im~ging purposes
are now well esta'blished and already find application
as image sensors in experimental television cameras.
Charge coupled devices as firs-t proposed
were based on the storage in depletion regions and
transpor-t adjacent the surface of a semicond-uctor layer
of one conductivity type of discrete'packe-ts of charge
in the form of mlnority charge carriers. Thus ~hen em-
ploying an n-type layer the storage and transpor-t is of
holes. Such devices are generally referred to as surface
` - channel charge coupled devices. In a later development
improved efficienc~ of the charge transfer between ad-
jacent storage sites is obtained in a structure in which
the storage and transport is of maJority charge carriers,
said transport being effec-ted via the in-terior of the
semiconduc-tor layer. These devices, ~hich may sometirnes
be referred to as bulk or buried channel charge coupled
devices, are described in our United Kingdom Patent
Specification No.~,4~l4,183 (PHN.5~04). SUCh a device
comprises a semiconductor body havin~ a semiconductor
layer of oue conductivity type, means being present for
electrically isola-ting at least durlng operation the
semiconductor layer from the surroundings thereof~ said
layer having such a thickness and doping concentra-tio
that a depl,etion region can be obtalned -throughout the
- ' , ' ;~
-2- ,
.
PI-IB.32531
~096 496 6~ 197~
¦ tlricklless o:f l;ho semlcQnclllcto:r laye~ by mca:rls of arl elec-i tric .t`ield i~hlle avoldll1g brea:kdowl1. The ck~v:i.ce a:Lso com-I pri.ses nleans to locally introcluce in-to the semiconcluctor
laye.r in:format:ion :in the :L`orm of` charge consi.stillg of
ma~jority chargc carrier.~s ancl means to read-out said in-
format.ion elsewhere in the layer, an elcctrode system
being presen-t al; least at one side of the layer to capa-
citively produce electric fields in the semiconductor
layer by means of which the charge can be transported
to the read-out means via the in-terior of the semicon-
~1
` ductor layer in à direction parallel to the layer. Such
a device may be constructed for imaging purposes whereby
the local introduction of information in the form of
majority charge carriers comprises the generation of
electron-hole pairs by the absorption of incident radi-
~;~ ation in the vicinity of depletion regions formed in -the
~ ~ .
! semiconductor layer adjacent. the electrode system.
.~ . ,
:1 . In order to be able to use the known charge
,
~: coupled devices *or imaging purposes it is necessary that
i 20 -the photon energy of the incident radiation is greater
- ~ than the hand gap of the semiconductor materia:L. This
. enables *or example visible radiation with an energy
: greater -than 1.1 eV to be detected using a silicon CCD,
both the surface channel and bul~ or buried channel con-
~` 25 f`igurations being suitable *or this purpose.
~ For imaging oP an infra-red radiation pat-
tern charge co~pled devices have been proposed in various
differerlt *orms. In one~ so-called "hybrid", form the
functions of detection and signal processing are per-
forn1ed in sepa:rate but integratabl.e components by an
, ` array of infra-red de-tector elements individually con-
.nected to a si.licon charge coupled device shift register.
~: _3_
, . . .
.
;
6--l;-l9~6
;rhc functi.on o:f t~le ;:iL:icoxl CC']~ t:lli.s case :is tha-t of
cl Sigilc~l pl''OCeSSO:L' pc:LLo~ ing al>l~:ropl:iul;e fnnct:io:ns. :Cn
anol;her, so-called "monolithic"~ .t`orrn the :fmIlcl;:iorl-; o:f.`
detection nnd si.gnal..process:illg a:r~ per:Lorllled :in the same
1 5 par-t of the semiconduc-tor body. Ln one proposecl form of
"monol.ithic" CCD ~or i:n:~ra-recl imag:i.ng -the operation and
s-tructure is essen-tia:Lly sim:ilar to that of a silicon
su:rface channel CCD and is based on the generation of
~ depletion regi.ons adjacent -the semiconduc-tor surface
1 10 where photogenerated minority charge carriers are col-
lec-ted. The device differs from conventi.onal silicon
~l imaging CCDIs in respect of the material of the semi-
¦ conductor layer. This has to be chosen so tha-t the ab-
~ sorption peak is in the infra-red region, the band gap
¦ 15 f the semiconductor material being less than the energy
¦ of the infra-red photons. Therefore the ma-terial is
restricted to cer-tain narrow band gap semiconductors
among the binary and ternary III~V 9 II-VI and IV-VI com-
pounds. This to a certain extent is disadvantageous be-
- 20 - cause the materials and processing technology is not so
~: - well developed as in silicon, it being appreciated that
whilst the basic charge storage and transfer operation
of a CCD need not be dependant upon the presence of a
. ~-n junction, it is nevertheless desired in many prac-
tical embodiments to incorporate regions of opposite
conducti-~l-ty type to the layer in which charge storage
¦ and transport occurs.
. In another proposed "monolithic" form of
1
I : infra-red CCD imaging device the operation is based on
the accumulation of majority charge carriers (electrons)
at the sur~ace o~ an n-type silicon layer and -their trans-
fer adjaceIlt the surface, said elect:rons being released
-4-
:,
~'
PlIB. 3~53 1
6~ I 97~
:f` l~ o~ d (~ ll o r :L ~ v ~ :r (~ l c~ L~ ; c~rl ll- t y ~> e
` s:i.l.icon l~ c~r. Op~:r~t:ioll ~.r t ~ 3 (10 V i c e :r~ -~ U i L~ C) O ~ r
i to a lo~ temperatllre :in or(ier -to m.ill:i.rllise~ the -therlrlal
gerlerati.orl of Ina jor:i-ty charge c~rr:i.e]rs ancl the inheren-t
d:is~advnll-tages of a conventional .surface chclnnel :in te:rms
of chclrge transfer effic:ieney and speed of opera-tion are .
' I acceIl tuated .
Il According to the invention a charge coupled
I device for eonverting an eleetromagnetie radiati.on pattern
i 10 in a certain wavelength range lnto elec-trieal signals
I eomprises a semiconduc-tor body ha~ing a se~iconductor
layer of one eonduetivity type in whieh pattern information
in the form of diserete paekets o* majority charge car-
. riers can be generated and transported vi.a the interior
of the semiconductor layer to means for reading the
charge, the layer comprising a coneentration of at least
one doping impurity characteristic of the one conduc-ti.vity
type and a concentration of at least one deep level im-
~¦ puri-ty as herein defined which provides eentres fo~ the
~l 20 trapping of majority charge carriers ~hich can be released
.l upon exeitation by radiation in the said waveleng-th range,
- the doping impuri-ty eoneentration and the deep level im-
purity concentration being provided so that depletion
~ regions can be formed e~-tending across the thickness of
`` 25 the layer while avoiding breakdo~n only as a result of
substantially all the deep level centres within the de~
I pletion regions being full of majority charge carriers
¦ charaeteristie of the one conductiv:ity type.
Referellce herein to a deep level impurity
is to be understood to mean an impuri.ty which is in a
~;l given charge state in -the bulk semiconductor material in
-thermal equilibri.um and whose charge state changes in a
32 5
T~q~6 G~ 9
el~](3t:ioll r(~;.io~l :i.lL ,1 C~ ncly ~ to. ~ ;c; r~ s l}l~-t t
c~r~ v~. 0.~ (-L ~l~ rlr)~ .ir~ ;Y~ ItlL~I;c:r~:i..l:l.
m~st be beLow the 1~erml lcvel (of` the scm:icc-llclllctc):r Ina-
t,er:ical :in equi.llbri~ l) .in the uE~per l-lal.:~ of -the blnd
gap and t;he energy :level Or a deep level. i!llpur-ity :in
~-type mater:ial must; be above -the ~ermi :level (o:E t:he
semiconductor mater;.al in equ:i.l;.briùlll) in -the lower half
of the barld gap.
Such a device in which the charge transport
takes place via the interior O-r the semi.conductor layer
and in which the generation oI`-the cha:rge packets is based
. on excitation o~ majority charge carriers trapped in the
: deep level centres and their release înto the potential
minima in -the layer rather t;han -the normal generation
of electron-hole pairs by absorption of radiation has
significant advantages when it is desired to form a mono-
- lithie eharge coupled device for converting a radiation
pa-ttern in a particular wavelength range into elec-trical
~' signals. Thus al-though in mos-t instances cooling of the
semiconductor body will be required it is no longer
~1 essentially necessary to employ a semiconductor material
-~l having an energy band gap which is 3ess than the photon
~ ~ energy of the incident radiation to which the device is
,~¦ to be sensitive. In particular -the advantage arises that
¦ Z5 for an infra-red imaging charge coupled device it is
possible to use silicon with its inheren advanced. tech-
nology, the deep level impuri-ty concentration being chosen
~ j to provide a sensitivi-ty -to infra--red radiation in a
:
: cer-tain wa-velength band.
In a device i.n accorclance with -the i.nventior
the deep level impuri.ty concentration in the .Layer wi].l
be of a value in excess of the value corresponding, .~or
, ~ ' ,, ' .
,
T
.` ,
.
969L9~ 6-~
n ~tl~cr o:t`-~ C~ ic~ c~or rl1~t~r:i~:L ~ncl -tllicl~ ss
an(l conta~ g On.ly a clop:ing .i.lllpl:lL`:i'(;y co~lcenl;ral;:i.orl ch.~:~rac-ter:istlc of tl1e one condl1ct:iv:ity type, to that which pro-
ClllCeS thc3 1nax:i1l1un1 possi.b.1.e n1(lg1l:itllc1c? o:f ne-t charge in the
~bulk of the :Laye,l- wh:ich :in a stecacly stato stil:L enflbles
. the :~or1llat:ion o:E` deple-t:ioll reg:ions ex-ten~.l:ing across -the
'~ wholc thickness of the semiconduc-tor layer while avoiding
A ' breakdown, l-t is men-tioned th-lt at :Leas-t in this respect
; the device s-tructure is distingul.shed from previously
3, 10 proposed charge coupled devices operative wi-th the charge
~, transport occurring via the interior o:C -the semiconduc-tor
layer and said layer comprising rnore than one impurity
concentration. In prac-1;ice the said maximum possible
. magni.tude of the net charge -that can exist in the bulk
~ 15 of the layer will depend oll the particular material of
d the layer of the one conductivity type. llowever for a
silicon layer in a device struc-ture which enables the
-~ layer to be depleted from opposite ma30r sides the mag-
nitude of the net charge should be less than 4 x ~o1 per
J 20 sq.cm. As will be described in greater detail hereinafter,
;~ for efficient conversion of i.ncident radia-tion the value
of the-deep level concentration will be chosen to be of
such a value -that the rnagnitude of the net charge in a
steady state is co-nsiderably in excess of` said limiting
~3i 25 value$ that is in the case described for a device com-
~j prising such a silicon layer which can be depleted fro1n
~ opposi-te major sides the value of the deep level concen-:.-
tration will be chosen to produce a net charge in a
steady state considerably in excess of 4 x 10 per sq.cm.
. 30 The operca-tion of a device in accordance
with the invention is based inter alia on the ab:ility
~ -to be able to fully deple-te the semicondllc-tol- layer during
:. ,~ .
~ . -7-
, `. .
, ~.
1"11~.3,'53
6~ '197~)
a i)e~L':io<l 0~ r~'l(lill.l;'i.Oll :ill'l,t`g~ t'iC)I'I ~; t]lollt thc occllrrerlcc
dUL`.irlg cl peL'io(l ():t` integ;~al,i.on oL`:ra~l:iation ~:l nor
qIL~ ; UIrI ~ 011~l-;.. t;~ c:i~t;s ~nd ~,h~ t c}-l~lL~re ~:i-t;ll-i
~, 5 I:hc depl(~tioll re~iolls llas to be kept be:low the sa-id mlx:i-~
, mulil poss:i`ble ma~-rll:itl-Lcle. Obv:iously one coll:l.d cons:ider -thecase, wh:ich :is ou-ts:ide the scope of the present :inven-tion,
where the to-tal num'ber of :impllrities, tha-t :i 5 -tlle surn of
the dopan-t and deep level impuri-ties is .l.:im:ited so tha-t
ln a steady state the sa:id maximum possible Magn:itude of'
I net charge could not be exceeded but such a struc-ture wou]d
not be very sensi-tive and would be very slow in response.
The dev.ice struc-ture in accordance wi-th the
j invention :is based inter alia on the recogni-tion that
¦ 15 the-quantum eff:iciency of the device :is dependant upon
the number of majority carriers trapped on deep level
~, centres incorporated in the layer andby providing a very
large number of the said deep level cen-tres and in opera-
tion integrating over a sufficiently short period -that
the number of such centres which release a trapped majority
carrier and theref`ore increase the net charge in -the
depletion regions does not cause the said maximum possible
magnitude of net charge in the deple-tion regions to be
exceeded, a high sensitivity consisten-t wi-th bulk or
buried channel charge coupled device operation can be
obtained. Theoreitucally if the number of deep level
cen-tres were limited as already mentioned so -t-ha-t even
1.
in a s-teady state -the said max:imum possible magni-tude of`
net charge could no-t be exceeded then one could have a
, 30 very long period of in-tegration. However with such a
,~ relatively small number of deep level cent,res to in-ter--
cept -the r~diation mo6t of -the radia-tion input would -be
, .
-8-
,.,
:1'1 IB . 3 ,' 5 3 1 C
~ 9~ 6
was-ted, that is not i.ntercepted. by -the~ deep level centIes.
Tile c~1)el-a-tloll o~ a de-v:i.ce in accorcL~ Ge ~
~iL.:11 -t;he :in~enl;:ion :is al.90 t~asec:1 Oll tile rOqUi.:l:'emO:llt tllrlt
lher111ally :i.nc111ced c11ar1ge-; :in t11e chargo stfltc o:t` the deo
:Le-~rol. ce1l-1;:rc!s occll.r ~1't a slo~er ra1;o -l;han the op-t:ica:L:Ly
:illldllCeCI ChilngeS9 l;hLlt :i.S the geno:ra-t ion. w:L-tll:LJl -the de-~
plet:ion regi.ons of free major:i-ty ch'c~rge carri.e:rs 'by
radlatl.oIl exci-tat:io:ll m1l.st be a-t an app:rec:iab]y h:igher
~¦ rate than by ther1nal excitat:ion. This to a cer-tain extent
will depend on the choice of the parti.cular cleep level
~¦ impurity and temperatllre of operation as will be des~
~ cribed in greater detail herelnafter.
'1 Various possibi.lities exist for the pro-
vision of -the doping impurity concentration and the deep
~, 15 level impuri-ty concentratioIl. In one form the deep level
~ impurity concentration is greater- thaIl the doping im-
¦ ' puri-ty concentration. In such a form the deep level i.m-
purity concentration will comprise a deep level donor
in an n type layer or a deep level accep-tor in a p type
:Layer. In this form the charge state of the deep level
centres in the depletion regions in equilibrillm is neutral,
namely a donor cen-tre having a single trapped elec-tron
' or an acceptor centre having a sing]e trapped hole is in
~: ~ a neutral state~ and may change respectively to a posi-
~: 25 tive or negative charge state by optical excitation. It
:~ ~ay also be possible to use centres which can trap two
carriers and the opt:ical excitation be such as to cause
:~ loss of only one carrier from'the cen-tre, these being so-
called dou'ble~negati.ve or doubLe~pos:it:i-ve cen-tres.
In another form' of the device in accordance
with the invention -th.e doping impurity concentration is
~ , greater -than -the deep leve] impurity conce1ltratioll which
,. :9 ..
- 9 -
. ~ .
~:
~3~6fl~3:~ G-1;3l976
:is l~ro~i(led as a Co~ cl1C;lt,~ g :il1lpl1ri~y. Ln such a form the
deep level im[nlrit~ co1lcentrat:ion l11ay coml~r:ise a doep
levcl .lCCe ptor :i1~ m n--tyl)Q lu-ycr or a deQp lc~(3 1 donor
~ :iIl a ~-type lay-er. :[n th:is fo-rn1 -the~ charge state oL`-the
i , 5 deep le~rel centrcs :in the dep:le-l;:ioll regiorl:in equ:il:ibr:ium
respec-t:i~re:ly is negat:ive anc1 positive, nall1ely Qll acceptor
centre l1aving a s:ingle trapped electron is in a negatlve
charge state and a donor centre having a sing'le trapped
I hole is in a positive charge state, and may change to a
1 10 neutral s-tate by optical exci-tat:ion. In -this form the
I posslbil,ity may also exist of us:ing impurities which
I yield so-called double-negative OI` double-positive centres.
! In some forms of the device the deep level
impurity concentration is formed by at least one deep
level impurity elemen-t introduced into the crys-ta:L lattice
1 of at least part of the layer of the one conductivity
!, type. Various elements may be used depending on the
~ semiconductor material and the wavelength range of the
radiation but by definition the response will be confined
'' to a wavelength range corresponding to energy values less
1 than the energy band gap of the semiconductor materialO
¦ In other forms of the device -the deep level
1 impuri-ty concentration is formed by defects in-troduced
;1 into the crystal la-ttice of at leas-t part of'the layer
f the one conductivity type. Such defects producing deep
~I
1 level centres may be formed by radiation damage, for exam-
I ple by proton or electron bombardment. The de:~ects may
1 act as compensating centres.
I The layer of the one conductivity type may
comprise a f:irst portion in ~hich transpor-t of free majority
I charge carriers can be effect;ed and a second por-tion in
hich genera-tion of :~ree ma jority charge carriers can be
~1 .
-- 1 0--
:
PlIB~3253l C
9~ 4~ . 6111-1976
eff`ect;ecl b~r r~dintloll :in lhe .;aiA wavelength ran~;e, the
cleep levc:L imp~lr:i-1,y corlcelltr~:ion be:ing, conr:i.rlecl s~lb-
stanti,ally to the socond port:ion of -the :layer. In sucll
a form the trnnspor-t ol` charge carriers takes place via
an inte:r:ior part of the layer ~hich is substantially
separated I'rom the part of the la.yer where the free
charge carriers are geIlerated by radiat:ion exci-ta.tion.
IIowever with:in the scope of the i.nvention there are also
devices in which the deep level impurity concentration is
~' 10 not localised in the said manner and the charge transport
, takes place via an interior part of the layer where the
I deep level impurity concentration is presen-t.
¦ When the deep level impurity concentration
is confined substantially to a portion of the layer,
namely the second portion as in the above-described form,
: and when such a deep level impurity concentration is
provided as a compensating impurity concentra-tion then
¦ ~ the doping impurity concentration in the remainder of
the layer of the one conductivity type, namely in the
first portion in.the above-described fbrm, must be of a
I sufficiently low value that the said maximum value of
¦ net charge is not exceeded.
.¦ Various configurations of an electrode
~`¦ system, form:ing part of the device, with respect to the
location of the layer portion or portions comprising the
: deep level impurity concentration are possible. Thus in
~,; a firs-t form an electrod.e system for capacitively pro-
ducing electric fields in t'.he semiconductor layer by'
¦ means of whi.ch discrete packets of majori-ty charge car-
i 30 riers as released. by e~citing radiation are introduced
'¦ into depletion regions and transported to the charge
~j reading me~ins is prcsent at one major side o:L` the layer
' ! -11-
, ~.
,. . .
~.
Pl113.3~531 C
~ ~9~ ~9 ~; G ~ 19 7 6
and t;lle s<~ seco:~lcl :L~y(r l~ort:ioll cOml):f:iL; ing ~ e deep
l ~ v ~ L` i. -t )~ C ~ C L~ . :L~ L ~ :L ~ 3 s ~ n -1; ll (I. j o ~ -i. r~ e
layer surf`ace at said one major side. This structllre
may l1e readily reaL:isec1 in manu~actllre and VL~ri.OUS
a:Lterna-tive Metl1ods of providing the deep level impL-Irity
concentra-t:ion may bc employec1 as will be descr-ibed here
inaf-ter.
ln a second form an elec-trode system for
capac:itively producing elecl;ric fields in -the semiconduc-
tor layer by means of which discrete packets of majority
I charge carriers as released by exci-ting radia-tion are
¦ introduced into deplet:ion regions and transported to the
¦ charge reading means is present at one major side o~ the
¦ layer and the second layer portion compr:ising -the deep
¦ 15 level impurity concentration is present adjoining the
¦ layer surface at the opposite major side of the layer.
¦ In this form the situation of the electrode sys-tem and
~I the second layer portion at opposite major sides of the
1 layer enables other struct~lral and manufacturing features
¦ 2Q to be advantageously employed.
! In one example o-~ sald second form an en-
hanced charge handling capaci-ty of the device is obtained
with a structure in wh:ich the layer of the one conduc-
tivity type comprises a more highly doped surface region
extending adjacent the one major side said more highly
doped sur~ace region extending over only part of the thick-
1 ness of the layer and spaced from the portion comprising
I the deep level impurity concen-tra-tion. For further ex
planation o~ -the mechanism whereby the charge handling
¦ 30 capacity of a buried or bulk channel charge coupled de-
vice is increased by the provis:ion of such a more hiphly
doped surface layer reference is :invited to-~
- 1 2 -
;i .
.
PHB. 32,531C
our Canaclian Pa-tent 1,003,939 which issued on January 18,
1977.
In bo~h the said first and second forms of
the device the layer of the one conductivity type may be
present adjoining a semiconductor region of the opposite con-
ductivity type, the electrode system being present at the
major side of the layer remote from the region of the oppo-
site conductivity type. Said region of the opposite conduc-
tivity which ~orms a p-n junction with the layer of the one
conductivity type serving for at least part of the isolation
of the layer from its surroundings in the operation of the
device may be present as a substrate of said opposite conduc-
tivity type on which the said layer of the one conductivity
type is present. ~However in one example of the said second
form of the device the region of the opposite conductivity
type is present as a surface layer, for example a diffused
surface la~er, at the` said opposite major side of the layer
which in this example constitutes ~he semiconductor body. In
; this form it is possible to provide the deep level impurity
;20 concentration in a portion which extends over a substantial
part of the thickness of the layer and thus, by virtue of the
large number of deep level impurity charge trapping centres
thereby provided, enables a high detectivity to be obtainey.
In on~ example of a charge coupled device in
accordance with the invention in which the deep level impurity
; concentration is sreater than the doping impurity concentra-
tion, the layer i5 of p type silicon, the deep level impurity
concentration comprises at least one of the elements indium
and thallium and provides a sensitivity to infra-red radia-
tion in the wavelength band of between 3 microns and 5
microns. In another example in which the deep level impurity
concentration is greater than the
.
~ - 13 -
`Q~
~,,
~ 5~1 C
~ 6 ~ 1976
, do~ g :ir~ r;ty collce~ltratioll t;llo layer :is o:f` ~-type sll:i-
CO~ SeCo~ l :imp~l:r`.i l.y COllCOnt:rat:i.O:Il :i.s o-L` g.~ clnd
pr(>v:icles a sensitivi-t~ -to ~ r~ rod rad:La~:ion in t]le
wave:Len{~;-tll l)alld Or bei;\~e~erl 8 mic:rons allc~ m:i.c:rons .
l~hcrl UsiIlg an n---type S:i l:i CO:tl layer in a
device in wll:icll -the deep leveL :impurity concen-tra-t:ion
is grea-ter thall -the doping :impur-ity concentr~t:ion then
the deep level irnpur:ity :is one wh:ich y:ields donor states~
One such example :is sulphur.
Reference has already been made to de-v;ces
in accordance with -the inven-tion in which -the doping im-
I purity collcentration is grea-ter than the deep level im~
¦ pur:ity concen-tration whicll is provided as a compensating
,~ impurity c,oncentration. Such a compensating impur:Lty
concentra-tion may be provided by acceptors in the upper
half of the band gap or donors in the lower half of the
¦ ' band gap. These impurLties may be dopant impurities or
defects but must have energy levels located a suitable
distance from the band edge, namely the conduc-tion band
1 20 edge in n-type silicon and the valence band edge in p-
¦ type silicon, to give the desired response -to radiation
', in the particular wavelength range for which the device
I is destined for operation. When using a silicon layer one
sui-table compensating impurity is gold which has a com-
~ 25 pensating level o~ 0.35'eV from the valence band edge in
'~ p-type silicon and pro-vides a sensitivi-ty -to radiation of
wavelengths from 1.1 micron to approximately 3.5 microns.
Gold also provides a compensating level of 0.55 eV from
~, the conduction 'band edge in n-type silicon and provides
a sensitivity to radiation o~ wavelengths from 1.1 micron
to approximately 2.25 microns~ As an a~ternat:ive compen-
,, sa-ting centres may be provided by de~ec-t levels produced
-- 1 4 -
9~; 1'1113;3,'-31 C
b~ t;io~ '; (? . I~ t` () ~; O ~ X C~ 1 ] ~ I. I.I C' ~ '
COmpellsat:.illg CeJlt::l~CS :i.n n-ty~ S:i ] :i.COIl ~i tll a .I~vo:I. o:E`
pprox.i.lnatel~ o.Ii eV :`.rom the cond-Ic-t:ioll baIl~I edge a:rLd
provi(le a SellSi. t:iV:i ty -to .rad:ial;:ion of wa-vel.engtlIs :~rom
S 1.-l Ill:icrolls -l:o aI>-I~:rox~ ate:ly 3 m-i.c:rol1.s.
T:Ile deep 1.eve:I. :impIlr::itv concentra-tio1l rmcly
be p:resent; as an ion inIp:LantecI concentration. T:he -use
o:E` ion iMplantation t.o provide 'the cIeep leve:L impur:ity
concentratioII is advan-tageous because ~it enables an
impurity eLement or elemen-ts to be provided unif`ormly
~ distr:ibuted over a g:iven area, which is hlgh:ly desirable
¦ in an imagi.ng device, and tobe precise:Ly confined in the
portion of the layer where desired cons:istent wi-th pro-
l viding sufficient majority charge carrier trapping cen-tres.
i 15 This conf`inement may be important in those forms of
the device in wllich -the opera-tion may be degrraded by the
presence of any charge trapping centres in the interior
~ part of -the layer where charge transport is effected.
¦ ' Furthermore ion implantation. being a non-
equili'brium process enab:Les the in-troduction of impuri-ty
elements having a normally relative low solid solubility
j in the semiconductor materi.al.
As tha optical cross-section of deep level
~ centres will be small, to provide the free majority charge
s 25 carriers by radiation excitation it is desirable that -the
value per sq. Clll. O:r the deep level impur:i-ty concentrat:ion
centres ~hould be a-t least 5 x 10 and even as hi.gh as
1 10 per sq.sm~ Cn prac-tice such a concentration. may 'be
r diff`icult to achi.eve in a thin layer because of` limi-tations
of the sol.id solubil:i-ty of the deep level :i.mpur:i.ties in
the particular semiconduc-tor materia:L. However -~hen usillg
high concentrations the p:roblem ari.ses that if -the charge
.s
~1 -15-
.
1'1113 32531 C
~ 6 6~ t~)
state o.L` a sllb;l;.llltia.L f.la(t:i~ ol` the cleep :le-vel impur:i-
ties :ii cll<ulgcll :i.n l;llo del)l..ol;o~ 'O~',`:i.O:tl~ tllorL :Lt wo~
rlot bc poss:ible to :L`uL:Ly (-I(:~p.Lote -tlle senl:icol:L(II.lcl;o:r laye:r
wlli].e a~roicl:i.ng brenlccl(lwn ancl sucll laycr dep:le-l;;.oJl i'3 a
lasi.c requ:il:~emellt;:fo:r -tlle sati.s:tclctory clla:rge t:ransf`er
opera-tioll ol` tlle clla:lge coul~led clev:ice. ~I:n. prac-t;-ice when
uslng such lrigll concellt:rat:ions th:i.s problem :is a-voided
by con-trolling -the integrE~tion period, that is the period
7 :i.n which the radiation can be incident on a par-ticular
. 10 imaging elernental por-tion Or the layer between successive
~ refreshing steps in whicll the deep level centres are
replenished with rnajority charge carriers~ to be such
:~ that in said peri.od with maxi-rnum radiation in-tensity not
:~ more than a certain number of the centres have their charge
state changed, for example not more than 1012 centres per
sq.cm. in said integra-tion period.
Thus in the opera-tion o~ a charge coupled
clevice in accordance with -the invention it is necessary
to periodically refresh the semiconductor layer portion
containing -the deep level impurity concen-tration by way
of replenishing the deep level centres with maiori-ty char-
ge carrier.s.
The semiconduc-tor layer may ~urther comprise
elec-trode means for enabling the periodic replenish-ing of
the deep level cen-tres with majority charge carriers. :Cn
other forms the replenishmen-t may be e:f`fected, as ~
be described hereina:f`-ter, by suitable configu:ra.tion of the
~, circuit mea.ns used for -the device operation.
Accordi.ng to a ~urther aspect O:r -the in-
vention there is provided. an arran.gemen-t comprlsing a
: ,
c:harge coupled dev:ice in accordance with -the invention,
circuit means :for supp~y:ing periodi.c signals -to a.n
, ~' .
~ .
! - - 1 6-
. .
~`
:~ .
I
~9~ 3 ~ 6ll~;3253~ ~
elecl,.ro(le ~Y~ it(`lll ~S ;Ot~ic~l;ecl ~ii.l.h t;l~e kly~r :I`or :r`o:rlr~ l,gd e 1~ l e ~ L` C' ~ S ~ t' '; ~ l :1. C ( ) ~ l C 1; O ~ Y ~ T; tlli.:~ i.el
d:j 5CI~e te packots of majC~:r:i l;Y C11ar~t`tt ea:rr:i.t3:rs as :r,e:l.eac3c(l' by e.~c:it:i.n,~ :inc;.(lerll::r.lcl:i.n-t:Lon e`rom -tho cent::res p:rov:i.ded
5 b-~Tr t;he deep lt~vel. :impur:it;y eonGentrat:i.on çan 'be co~Lected
and l;rallspoltecl to the :rt~3ad-l.ng Irleallcj :in cl d:i.reet:i.oll
parallel to -thc laye:r v:ia an lntt3r.-i(>r par-t o:f the layer,
and circui-t rneans for enabling the period:i.c replenishlng
.i of' -the cerltres provicled by the deep level i.mpurity con-
cen-tration with majori-ty charge carri.ers.
I.n a first form O:T' such an arrangement the
~'. circuit means fo:r enabli.ng the periodie replenishing of
~'i the deep level impurity cen-tres wi.th majori-ty charge
carriers comprise means I`or discharglng the depletion
i 15 regions whieh also extend through the part of the layer
;~ eontaining the deep level impurity eoneentration.
z
In another form of the arrangement the
semi.eonduetor layer comprises an input stage for the
generation of pacl~e-ts o-f majori-ty eharge carriers whieh
,,
::~ 20 ean be transported in a direetion paralle:L to the layer
rl!. and the circuit means for enabling the per:iodic reple-
nishi.ng of the deep level eentres with majority charge
' earri'ers eomprise means for applying signals to the in-
put sl;age for periodieally introdue:ing re~`reshing eharge
.~
paeke-ts of majority eharge earriers of such magnitude
that as they are transported through the la-ye:r -they
ex-tend at lea.st into the part of -the laye:r con-taining
~ ' the deep level impurity coneentra-tion.
':',
'¢mbodiments of the invention will now be
.~ 30 deseribed, by way of exalllple, with reference -to -t~e a.e-
companyin,,r di.agramlllatie drcawings, :in w}l:iell:
. ~ .
;~. -17
i -
~,
j .,
. ~
6~96 I'lll';3253l C
l~`i gllre 1 i .s n pl.r:~II V.i C-`W o:t` pcar~t of a chargc
(.`OIII~leCl do\ricc :i:ll aC(.`(>I'CIan(`e W:itll tl10 :LIIVetJI;:;O11 ~
:iS sllitclL)].C :I`or :i.nlagilLg a-n -in:t`ra-le(] racl:ia-l;:io:n p:.ttern~
l;`:igu:ro 2 :iS ~-L sec t-i..o:na L V:ielr o:t` -the dev:;.ce
silo~r:n :in :F:i.gll:re 1 -tak:ell along the :Line :L~-II of`:L~igl:l:re 1,
Figl-lre 3 i.s a secl;:i.onQL v:ie~ of -the clev:ice
slLo~n :in l~:igù:re I -laice.ll along the l:ine [:[I-:[:[I of :F:igllxe 1,
F:igu:re ll i.s a sec-t:ional view of ano-tller
charge coupled devi.ce in accordance w:i-th the inven-tion
which is suitable for ima.ging an inf`ra-red rad:iation
pattern~
Figure 5 is a cross-sect:ional view of the
device shown in Figure ll taken along -the line V-V of
Figure 4, and
:l 15 Figure 6 is a diagrammatic representation
~ of the wavefo:rrns of the potential app].ied to -the elec-
. , ,
~ trodes of` the device shown in Figures l~ and 5 when in
.~ operation for imaging an :infra-red radia-tion pattern.
Referring now to Figures 1 to 3 there is
~ 20 - shown a simplified form of embodiment consisting of a
¦ CCD infra-red imaging device of which the elemental
imaging parts are in the form of a linear array. Ob--
~ viously wi-thin the scope of the invention are also de-
¦ vices in which -the eleniental imaging parts are in the
form of a two-dimensional array, a so--ca].led area imager,
bwt for the sake o:~ clari-ty of` illus-trati.on and descrip-
tion of the dev:ice structure ln accordance wi-th the :in
~ Y vention the linear array embodimerl-t of Fi.gure 1 to 3
; ~ ~ will be descrihed.
The device cornpri.ses a sem:iconductor hody
~s 1 of silicon ha-ving a ~-type semiconduc-tor :I.ayer 2 ad-
joining the surface 3, sa-id layer 2 be i lLg present on ian
.
~,
i: ~
~';, - 'I
,
3~ IG~ 5 3 1 c
~1~ t,~ . 'r~ ,$ ~, L`~ f ?()~ i{:ro.ll~;
t.lli C`~ (?`-S l'ill(l ll;:~':i ~1 .1.'(`'; i :i l;:i V L l;y O.l. 1[~ Cllllll, Clll. ~rllc, l~lye:r
:is oI ()Il):i C`l`(>n~i tll:i C~i:lLe';'; ~-nd COlnl).r:i .c;e'; a ';I~ I.'f`aCC~
jOi..llillg p(Jrt:i()n; 5 in w1Lictl t1~ rc--? -is .an :;on l.lll~plantecl
col~c~ ;r~ :ic)ll o:f` i.~ l Tll~ 1ayo:l 2 :i.s rlop(cl ~i.tl-L l~c):ro
:in a sul~sl;nl1.t~ l1y -un:i:rc):llll oollcelltrat:ioIl Or 1ol5 at;olns
: cm 3, tllat :;s approxilnat(~ly 6 ~ l01l ;mpu.r:i-tles por sq.
Clll. in any plnne o:c` the layer. Ihe ion :implanted concen--
tra-tion of indium extends -to a dop-tll f`rolll -the surf`ace
3 o:~ 2 microns alld the peak concentra-tion :is at a clepth
~ oI` 0.25 microns from the surface. The dose used for
i forming the ion :implanted concen-tra-tion of indium is 5 x
~ 101 per sq.cm.
¦ The n-type suhstrate l~ together witn the
~-n junc-tion be-tween the substrate 4- and layer 2 f`orm part
of means for isolating the semiconductor laye:r 2 from
' its surroundings at least during operation of the device.
I Said lsolation mea:ns addi-tional]y comprise an insulating
layer 7 of` sillcon oxide on the surface 3 and an n-type
surface region 8 which as shown in the plan vie~ of Figu.re
1 la-terally surrounds the p-ty-pe layer 2. In t]le present
embocliment the region 8 is an n~-di:[`:~wsed region which ex-
tends throughout -the thiclcness of the layer 2 and into
~i the substrate ~L. ~or the electrical isolation of the
layer 2 the ~-n junction between the region 8 and the
layer 2 may be suitably reverse biased, the connect:ion
-to the region 8 being via a connec-tion to the substrate
4. In other embodirl1ents it is possible to employ ins-teaci
~:~ of the region 8 a sunken insulating layer or to have an
r
J 30 n-type region which extends :~rom t:he surfaee 3 of the layer
over only part of` the th:ickness of thr3 layer 2. ~l thr
lattr.~r case opc:rat:i..on is e:~ec-ted witll such applied ~poten-
, ' '
-19-~
v
~s~ 6 ~ j 3 ~ 5 3 1 c
-ti.Il5 tlka~t thc dc~pI.~t:ion l~Og:i.OIlS as.0c:iLI.tf3cl w.i.th t~le sllb-
~t;:I~t~ .s:r.~ t:i~ 'C(:!:tl Lll(l rl.-~L.y~ ll:L'.~'.lCe
re~,.;oll nllcl t;l~ t~po layeJ- ,' comp:l.c1 I;cs tho i .~;o'l.at;:i.c)n.
'I`hcs -l;II:ic:I;ness ancl the boron dop-ing of tlle
~--t-Srps-s :kl~er 2 ls such L;hcl.t; by nIec~ s o-'.` aIl f;-2l$~$C tri.C
*:iolcl a df~2pl e t;:i o:n rfcgioll Call be f'o:rm~scl ex-tend:ing -througll-
out the t~ -ickn~sss of the 1.a$r~sr 2 wh-i'Le avoid:i.ng 'b:real~-
clow~ r usi.ng such a laSrer 2 which :i.n operation can be
~! i.so:Latecl *rom the surroundings chal-g/3 in the form of
s ' 10 majori.t~r char~e carri.ers can be transported mainly i.n
~ particular the last fractions of charge packets to be
~1 moved between ~torage sites v:ia the interi.or Or the
`.~ layer.
s) The eharge which forrns pattern :infcrmation
s 15 is :introdueed into deple-tion regi.o:ns formed in the par-t
of the layer 2 situated below the surface portion 5 from
¦ ~ deep aeersptor levels prov-ided by the ion implantated im-
Z puri-ty eoneentration of indiurrl. At -the ternperature of
. s
~ operation substantially all these impurities are un-
~-s
3 20 ionised and the sald deep acceptor levels form trapping
~: centres for holes whieh can be released upon excitation
;~ by infra-red radiation in the wavelength band of 3 microns
~., s
to 5 microns. The extent of the sur*ace port:ion 5 and the
' applied potentials at various parts of the dev:ice as will
.~ ' 25 be described herei.nafter are such -that the potential minima
~ in the depletion reg;.ons formed in the la~er 2 wi:Ll be
'-~s situated at posiI;:i.ons spaced :t`rom th-3 surrace portion 5.
~ ~ :
~-~ Thus provided will be situated at posi-tions spaced from
the sur:''acc~ port:ion 5. Thus provided the size of a pattern
inforn-Iat:i.o~l represeI:l-tin.g charge pac~et doe0 not exceed a
cer-t'ain value as can bfs de-termined 'bSr other operational
i:,
'3 :t'eatl1re.s as will be described 'lilereinc-I*ter the transpoS-t
;li -20- .
:,
~96~96 ~il13.32531 C
6-11~1(376
oi` t.~1o cllargo 1~a(~kel;S r~-J:~ose1lt;.~t.ive of 1~atterll :in:f`ormcltion
C l r~ c~:f~ cl.~(l \~ l1 L :i. I I ~; L~ :I :i. O ~ 7 cl X t of tll~, I.,IYO:L w:itl~ t,
t11(` c}laL~;c~ c1~ts t i :~ y ~ ; O ~ 1 ( ' S l;l r` f ~ C ~ O :I~ ` -
t;:iOI) 5 cont.ai.ll:ing t:ho t:rl1-1):LIlg cent;re~s as l):rov.icled by
S t.he :in1p:Lal--l.ed concellt;rltion o:t` inc1.:iun~ hus p.--t-terll:i.n-
:L'o.lln.l-l;:ic)l~ the :forln of d:iscrete packo-ts of m.i;jor-ity
.lr~;e Car:l';.eI`S iI:L S tOrclge S.i tes i.n tlle layer 2 can be
gencrated when :klciclellt infra-red :rad:i.ati.oIl in the saLd
wavelengrtll band penetra-tes t,o -the surface portion 5. In
the presen-t embodimellt in which the layer 2 ls doped with
acceptor impurities, both in the mf~in bulk of -the layer
and the surface por-tion 5, this means -that the pat-tern
information generatecl via the deep level centres, intro-
ducecl into storage s.ites in the layer and therea:fter
transported through the .Layer :is in the :form of holes.
i At one end of the layer there is a ~nore
;~ highly cloped p-type surface region 10 on ~hich an input
eomlection conductor 9 is present. ~t the other end of the
layer there is a more highly doped ~~-type surface region
12 on which an output connec-tion conductor 11 is presen-t.
- The input eonnection conduc-tor C3 and surface reglon 10
enable f'urther introduetion of holes into the layer 2,
~¦ thls lntroduetion and therea:fter transport through the
`1 I.ayer bclng effeeted, as ~ill be descrlbed here:inafter,
to periodically replenish the deep acceptor le~els, as
provided by the ion implanted concentration o:f lndlum,
ltll holes, The output conductor 11 and sur.face reg:ion 12
ls used as par-t of a means :for read-ing the size of -the
~~ O
:~ charge packe-ts and :for removal of the charge packets fron
the layer bu-t thls part of the operatlon ~rill not be
' described ln de-tclil as lt Ma~r be ef:fected in a conven--
j tional man:rler.
s -21--
, 3 2 5 31 C
~ 6~ . 6~ 76
On tllO Sll~ t`:lCe 01` the :LaYer 2 ~llere :is arl
elecl;l~ode systelll:lor cnpac:i.tive:ly gener,lt~ g clectI:i.c
fields :irl l:l-le :Layer 2 by melnS of wll:i.ch pat tCl.`}l iYlrOrrllQ~
ti.on represent~ lg ch.l.rge p~ckel;s can bo co:l.:Lect;ecl and
t;hereaftc7r transported to the read~ou-t means (11 t 12) iIl
a d:i.rect:ion pa:rallel to t;he l.ayer. The el.ectrode sys-tem
comprLses a large plurality of electrocles 13 .in the :form
~- of cond-uct:ive layers ~7hich are separated from the sem:i-
con7cluctor :Layer 2 by the sil.icon oxicle layer on the sur-
face 3. The device structure is suitably arranged wlth
respect to the intended direc-tion of inciclence of the
radiation pattern. Thus, for example, if -the infra--red
~-l radiation pa-ttern to be recorded is to be directed at
the upper surface then the electrodes 13 are cho~.en to
be of a material and thickness which will allow transmis-
sion of said radiallon, for example the electrodes may
be of polycrystalline silicon. Al-terna-tively when using
a silicon body of the said dimensions the radia-tion pat-
. tern may be direc-ted at the lower side of the body as
. , .
infra-red radiation in the said wavelength baIld will be
transmitted by the substrate 4 and layer 2 in reaching the
portion 5 containing the deep level impurity centres.
In a direc-tion perpendicular to the intended
direction of charge transport the electrodes extend across
the entire width of the senliconductor layer 2 as isshown
: in Figures 1 and 3. The device as shown iTl Figures 1 to
~ 3 is operated as a three-phase charge coupled device in
`;~. which the electrodes 13 are connectedin three groups to
clock 1.ines ~ 2 and ~3 for applying clocking voltages.
It ~7.~7ill b~ appreciated however that the el.ectrode con-
figllration in a device i.n accorclance with -the invention
;~. may be other than as showrl :in the preseTlt embod:iment,
. ~ .
-22-
.. .
3.32.5~31 C
6 ~ 9 7 G
.r O ~ X ;~ > I t ~ C~ t ~ ) C~ :f` O :rl1l ~ ( l 0 :~` ]) O L Y C :I:~ Y ~
s:i:L:i.coll at. d:L:I`:L`C:re111; LeVU I S a11d j.:11 a:n OVerI-ar)~ .:r1g rCI LU-
-t:ionsl~ , the electrodc?s mcly be p:rov:idod on :insul.lt:i.ng
' :I.ayor l)al-ts of` cli.:~:t`orenl; -tl-lic:klles.c;, alld o-ther l~nown rneans
: 5 Inc~y be elllp:loyed. ~urthe:rll1ore -tlle clev:ice nlay be COll'S trllC to(l,
for exanlp:Le, for t:~ro-pllase or follr-r)~lase operfttion.
For a fllll descr:iptiorl of the charge trans-
fer operation of a charge coupled dev:ice of the fo:rm in
,~ wh:ich the transport occurs via the :i.nterior of the seml-
..
~ 10 conductor layer reference is invi.ted to United Kingdom
? . Patent; Specification No. 1,414,183 (PII~Ts5476). The
operation of -the device sho~l i.n ~igures 1 to 3 in so far
¦ as it concèrns the formatioll of electrical signals re-
presentative of an infra-red radiation pattern will now
~ 15 be described.
;~ The substrate 4 is se-t at a reference poten-
~? tial, for example earth, while a voltage of approximately
~, -10 volts is applied to the layer 2, for example via the
;` ! input contact 9. The clock voltages applied to the lines
01~ ~2 and 03 vary for example, between -~20 vol-ts and ~5
I volts. Starting from the situation where the free majority
.¦ carriers, -tha-t is untrapped holes, are removed from the
¦ layer 2, i-t can be calculated that for holes potential
~? minima in the depleted parts of the semicond~ictor layer
;l, 25 below the electrodes l3 are obtained at a depth of appro-
;1 ximately 5 micron.s. In said potential n-li.nima charge pac~iets
? in the form of holes, released by infra-red radia-tion ex-
citation from the deep levels provl.ded by the implanted
concentratlon of indium in -the surface porti.on 5, can be
introduced. In any one elemental :imag:i.ng portion (bit)
defixled belo~.~c~ group of three ad;jacen-tly situated elec-
trodes l3 the ho.?.es released by the exc:i.t.i.xlg rad:i.at:ion
~ 2 3 -
,: ~
'' '
.
l~ 2 5 '31 C
~ 3~ 7 G
a~ c ~ e ~ t (~ cl :i tl t; ~ lC~ o ~ i. tl:i. lrl~ c (l ll c ~:~rl t; I~cl t ~
1 o ~ t 1 1 c O I1 ( ! O :~ ~ 11 (` C~ C l; :L~ O XC ~ . O 1;11(` O 11C C O I1--
ectecl to tllO 1i3~e ~3, to ~ icll tl~e LO~"e9 t vol.taee:ic;
C1PT)1;-ed. I3Y SUital)Le ChO:iC(3 O:f' L.}Le c:Lock. vo:l.tages tllexe-
a:'L`ter~ app:L:i.ed to the elec t,rodes sa:icl charge pac1~e ts as
collec tecl, :f`or e.~;alllple bo:Low the elec trocles conl1ec-ted to
I;he linc 03, can be transported to ca reg:i.on below -the
nex-t succceding el.ec-t:rodes and so on. :~n -th.i~3 maIl:ner -the
charge packe-ts ini tia:Lly collec ted uncler -the elec trodes
'10 connected to the line ~3 are sequen-t:ially transf'erred to
, the read-ou t mealls ( 1 'l, 1 2 ) where they are removed . The
charge transport of` the last frac ti,ons o:t` the charge
packe ts can take place at a comparatively large distance
I`rom the electrodes 13 and also f'rom t:he surf`ace portion 5.
The device is operated in a cycl:ical mode
wherein at the commencemen-t of a :~rame period the deep
~ level centres are first replenished l,~i th ma jority ch.arge
:~ ~ carriers . Thereafter in said period there is an iMaging
,,,'~ ~ integration period in which pa ttern in:~ormation. i9 con-
.~ 20 verted into discre-te packe-ts of charge present in depletion
!
~ j regions formed belo~r the electrodes 13 and i n each bit
,:~ collected in the depletion regions belo~T the elec-trodes
connected to the c:l,ocking line 03, sald charge packe-ts
.l being of a size dependent upon the to-tal local intensity
I ,~ 25 during the integration period of` the inf'ra-red radiation
i
incident in the relevant part of the sur:E`a.ce por-tion 5
-~} of the laye~r beIow the three elec-trodes 13 of~ th.a.t i.ma,~;:ing
,1 bit. Thereaf'ter in -the said frame period the pattern in-
,, i forma.tion representing charge packets are seque:ntially
trans:L`erred to the reacd~out means and remo~red from the
layer bef'ore the comrl1enceme:n-l; of' the ne~;t frame periocl.
i
: ~ .
~' 2
., ' .
l'llt.32531
9 ~ ~9 ~ 1976
.L`.i.l~ ." llr~ .l.y tll(-~ :L~ lL~
Or -l;1le deep :I.eve:ls ~ tllo s~ l'aco 1lorl:ic)ll 5 1~.i.l;l-l h():l.ei
is ~:s'`:r~c-l~?-l :i.ll -tl~:i.3 ~rll'ho~l:Llllcll~ y :i.M.t:t~o~ c~ ; all o~r~chn:rge pac'.lcet:s at -the :inT)I.l-l; (9, -iO) nn(l tr.ans:for:r:iLIg
tl?em -to t;}1e outp~ll. 13y tl~.e terln "ovo:r-s:i~e" t~Le:re is to
l~e ullderstood charge l~ack:ets of suc:h magnitude -that as
they pass -through the lr.~ye-r they w1i.1 'be located in par-t
. in the su:r:face por-ti.on 5 and -there donate ho:Les -to -the
unoccupied deep .I.evels In an alternati~re form the deep
levels are repleni.shed by connecting all the electrodes
13 simultaneously to ground poten-tial in order to dis-
~3 charge the dep:Le-tion regions inclllding the portions
t;hereo~ :formed in the surf'ace por-tion 5.
With the high photon ~lux occur:s~ing in the
inrra-red region of the specLrum in many ins-tances the
franle per:iod can be re]atively short, for example in
~ the case of a 100 bit linea.r array the franle period may
:'~, be 100 microseconds of which the integrati.on period is
95 microsecollds. The actual frame period w:ill be de-ter-
mined by a number of :~actors in any particular case, in-
cluding the n.ature and concentrat:ion of the deep level
.1 impurity. Thus the -.~rame period may be, :~or example as
3 long as 40 mi.lliseconds and as shor-t as 10 microseconds.
In any particular case using a silicon layer the net
charge wi-thin -the depletion regions cannot exceed appro-
ximately 1-l x 1012 per sq.cnn. since the breakdown f`ield
l would be exceeded. The duration for the :in-laging integra~
,~ ~
' tion peri.od par-to~ -the ~rame period is chosen in con-
junction wlth the concentration of the deep l.evel i~n--
purity so -that with infra-red radia-tioIl o~ max:imunl in-
.~ tens'i-ty i.ncident -throughout sai.d per:i.od on sarly o:nc ele-
T l menta:l. :imag:ing portion 'be:l.ow a. groupo~ elec-trodes 13
.~;,,
~ -25~
6 ~ ;3~5~1
:f`o~ o~ t; ~c; ~ tl~r~ c~ t;]lo
r~l. (,:t ~ lot~ e ~ :r.~c~ ;lol1 5 ~ L.l. l~o
:f`inec]. to ;a:l.cl p~rt al-lcl cll~r~:i.ll~r subse~ cl1t t:rarl;por-t to
-t;lle ollt~ llearls ~:il.l. no-l; e~toM(:l -to l;ll(? s.lr r~ce port:ion
5. COllt;a i:LI:i.ng the -I;rapp:ill~r ee:nt;:res.
t;}le:r elllbodimen. i, o:[` l;he :i n-ven t:ion ~
no~ be clese:l:ibed ~:ith ref`e:re~lee to ~ifJLL:res l~ to 6. In
thls c1eviee par-ts eorre~spond:ing t;o those sho~n in Fig-ures
:! 1 to 3 are :indi.catecl by the sanle re:PerQllee numerals. t`he
main dif`ference resi.cles :in lhe ~-type silieon epi.tax:i.al
layer 2 being of a mueh 6reater thickness and `having
diff~erent; :impurity di.st:rlbutions. ~hus the layer 2 is
of 35 mieron.s thielcness overall and contain:irLg boron
3 throughout its thicliness in a baekf,~round eoneentra-tion
of 1014 atonls em 3. The ].ayer 2 has a fi.rs-t portion l6
.~ of approximately 25 mierons thiekness provicled adjoining
`~ the substra.te 4 and GontainiIlg as a de~p level impurity
3 ` a eoneentration o:t indlum of 5 x 101 e~n 3 whieh corres-
ponds to a value of 1.25 x 1014 impurities per 9Cl. em.
` b
B 20 Adjaeent the surfaee 3 of the layer 2 and separated f`rom
the portion 16 eontaining indlum by a portion 17 con-
taining substantially only the baekground eoneelltration
. ~
~ of boron -there is a more highly doped portion 18 o~ ap-
s
..~ proximate~Ly 1 mieron -thlekness eontaining an additional
diffused eoneentration of a suitable aeeeptor, for exam-
ple boron having a eoncen-tration of 5 x 1015 cnl 3. In this
~ `
device the isolation is in part obtained ~i-th the aid
of -the n~-surface region 19, which extends onl~r par-tl.y
through the layer 2, by applying a SUi table re~erse bi.as
.~ 30 aeross the ~-n junct:ion bet~.een the region 1~ and the
!`. layer 2 that the depleti.on regioll assoe.i.a-ted ~ri`th this
junction extends at 1.eas-t -to -the depletioll regl.on assoei-
,'
~ 6-
l~3 3~531
9'~
~Itccl ~ i.s Jullc~ ~xt:(!llds lt :Lc.:lst lo l11~ deplel-io
l`~?~:i 011 .i'"-i()C:i ;~ d ~ ` :L`CVO:I`.Sl' ~ .s~ 11 JIII1CI-;.()IL
bet:wec~ e n-t)~lpC~ sllL)str.ate l~ alld -t;he ~-tyl,o .Lay~r 2.
Operatl O:tl of th-i.s dev:i(~ Illay be e:L`:L`ect;ed
~ a silll:i.L.Ir l~1a~ e:r to -that closcri.'be(l fo:r -thc prev:i.ous
onl~od:illlerll. I:rl th:is em~od:i.lllQnt IhC electrode struc-ture
is su:itabl.y :f.`orlllecl t;o a:llo~ transll1iss:i.orl o:t` inrra-red
radiat:i.on :in t:he wavelength band ol.` bet~een 3 m:ic:rons
and 5 microns. IIowever the dev:ice structure n1ay 'be
readily n1odifiecl to folm one in wh:ich the radiation
~¦ pattern :is d:irected from the subs-trate sicle.
¦ Figure 6 sho~s the waveform diagrclms of
. -the voltages applied to the lines 0l~ 02~ 03 and -the
~oltage of'the input e.1.ectrode Vj . In -th:is tnode of
j 15 operation -the potential appliecl. to the ~-region 10 nor-
¦: mally detern1inos the po-tent.ial applied to the layer 2
~i and ~ith respect to the substrate whi.cll is grounded is
-50 vol-ts. Fo:r replenish:i.n~ the deep level impurity cen-
tres with majority charge carriers the potential Vin is
temporarily rela~ed to the substrate potential. During
-this period, designated tri the deep level cenl;res pro-
vided by t:tle indium concentration are replenished witl
I holes due to the cleplet:ion regions in the la.yer bein~
I fully discharged. Followin.g the period -trl the CCD is
-~ 25 reset for a period indicated by trS during which s-ub-
stan-tially all free majori.ty charge carri.ers are rellloved
from the layer. A-t the enc]. of t:he per:i.od trS -the volta~es
on the lines ~11 02 and 03 are held at constant levels
for a per:iod ti :in ~hich i.tnaging o:~ the infra-red pa-ttern
occurs ancl the charge pac'k.~c3ts of ho.Les as generated, as
previous:L5r described~ by the e~:citing radiati.ol1 are col-
:Lectecl be1.o~ th.e e'Lectrod.es ~3 i.n each po:r-tion. Thereaf`ter
:~ .
--~7
6 1~ .3'531
~ 197(~
:i]~ L`~ 0~ i ocl l. -t~ C`ll ~:L`~ C~ C` ~ :L C` ~ I.
Oll t t C~ t tle (~l~ t p~ t; e l.(:~ C: I :I oclc~ ll.d :rc l.d i..~y IlI(`r~ 15 nO I;
sl1o~in rl1e c:Locl:i.~ ; vo:1-~;lges on t;hc l.i.:ne~ 3 ~:i.ll
L~e ChO~CIl aCCOI'CI:i.llg t;(i a lll.ln1bel.` 0:~` ~1.;:r:E`1-~:rent :L`actors~ a~
n1ay Vc~J.y :in ollo exa1l~ le betwec!n 5 vo.Lts ald ~5 vo:1.ts. :Cf
i.t :i.s de.c;i:LecL -l.o bring -t11e c1large packc3 l:s nec1rer to the
sur:race 3 then :lo~er clocl~ing vo:Ltages may be uscd, and
:in anotller example -these voltages ~i.LI. be negat:i.ve w:ith
respect to the substra-te.
It wi.Ll be apprec:ia-ted -that various manuf`ac-
~. turing techn:iques known i.n the art ma)r be employed in
:~i the :forrnat:ion of`-the embocli.1T1ellts so :~ar describecl and
in otlLer modlf`ica-tions as l~ill be descri.bed hereina:~ter.
i In the manuracture of -the embod;. :nt described with re-
;j 15 ference to ~?igures ~1 and 5 the ~-type layer comprising the
layer portions 16, 17 and 18 may be provided in two stages.
~:~ Thus the layer part :in which the deep level impurity is
l-o be provided may be f`irst provided by epitaxy on the
.~ t;
i~, substrate L~, thereafter the deep level impul-i-t jr provided
.:,
in said layer part by ion implantation or diff`usion, and
~J then the remaining layer part provided by a second epi-
, ,;
taxial deposition step, the more highly doped surface
portion 18 being provlded, for example, by a subsequent
~ dif`:~usion step. As an alternative to providing the deep
:. 2~ level impurity by ion implantation or dif:~us:ion the -first
: I .
,~.! stage epi-taxy may be carrled ou-l; to simultarleo~.Ls]y deposit
the deep :Level imp-urity ~ith the backgro1lnd. impllr:ity
~ elernent~ f`or example 7 ~/hich determines the conduc-tivity
.~ : of said l.ayer part.
~: 30 I-t ~/i.ll De appreciated that rOr th.e de.sired
, ! ol,eration Or the desc:r:i.bed embodi~nellts it ~il.l. he necessarv
: to coo]. the semicond1lc-tor body. Ii1e ac~ual -temI)e:rature of
., ~, ,,
.,
~ ! -2~,-
,~ .
~, ~
~9 ~ 3.3253!
6 ~ 1 9 7 G
c~ i t~ {~ f~ r~ (l I.))r ~ (' n~ c o.~ clc e l~
lc~ol :i~ it~. :I-I :i.~. ~i ! . ~ 11 I; :i ~ ~ 1. i. l l ,,l i; t l l O :L` ..I t ~ tII~ tl.l.
exc:i.I.;~I:i.on (,:~` tIe ~leop Iov-e:I. cc.Ilt:I-(S :i.s a~ roc.ilI~ly :Loss
t;ll-~rl i-~ L~al;~ o:L o~ i.c~ ~c:i.tili.orl. I`hus :~o:r l s:;l:lco
$ :;n~`:L~a~1~O~I im.lgilIg dev:ice :f`or oI)el-It:ion :i:n the 3 m:iCrC:Il
t;o 5 mi CrOIl b-an.cl cooling to 77l( should gl.vo accept;able
opera-t;-ion when us:ing a ~-chan.ne:L CCI) wi.tl-L tllall:i.urIl as -the
deep leve:L lmpur;ty. The already described altern.clt:ive
element :indium may necess:ita-te fllrther cool.ing. l~or a
silicon device for opera-t:ion in the waveband range of 8
~ microns to 14 microns cooling to a low temperature may be
1 required for satisfactory operation, for example cooling
-to 20K. However, SllCh coollng is considered t:o be quite
acceptable having regard to the fact that by use of the
device structure :in accordance w:ith the invention in
. which using an extrinsica.lly doped semiconductor layer of
a eertain ma-terial, for example si.li.con, the sensitivity
of the devlce is effeetively extended in.to a wavelength
band where the photon energy is substantially less than
the band gap of the semieonductor material.
ALthougll the embodimen-ts described are ele-
¦ mentary linear arrays :it wlll be appreciated that a device
in accordance with the invention may be of more complex
conf:iguration. Thus, a so-called area imaging device may
be formed and various means of reading the charge as used
in convent;ional imaging CCD s may be used, for example a
sensor may additionally cornpti.se a masked arra)r of ele-
ments correspon.dlng in number to the el.ernents used in the
imaging part of the sensor, the masked array be.ing connec-
~, 3 ted to the imaginr; arrca~ ancl serv.i:ng as a store whlch :is
reacl-ou-t through a series--pcal~al:Lel CCD coIlvers.i.on stage.
~, r~ro fu:r-ther embod:i.Inerl(s o:f tlle i.n-vention ~i.:L]
.~i '
I -29-
;'
,
~3~ 3~ . '3~'S3 1.
6-1 I- j~)7fJ
. n0~ bO (10S~L~:iLIe(~ lOSe ellli~O~I jlnCI1tS I.-( Lng InO(~ L`iC(It;;OI1S
Oi_` t.llt~ C~ t`lll Cl-`'i(`I.`i IJ(~ f`(l-~ c~ ;LI:L~O~.; 1
~c) 3. 1.1l th(l :t`i.rst: o~ n:i.d :L`I.lrl;lle:l onlhoclilllt~llts 't)l~' cl~ep
leve:l. llnp~ l.-l;y is pl~o-~:i(1e(l EIS a. (`Olllpen'3a 1; ill,"; (leOp levcl
c10llor i.l~ :r:ity Ihr~oughc)u-t; l;he :Layer 2 ~IL:jC11 is moro h-igl~
.. ly cloped ~r:itlh bor(:,:n~ n~ll1e:Ly a bo:ron cOnce~-tJ-~a-t-ion c-f
.` 5 x lO I at;OlllS Clll 3 ~h:i.cll is ap~E~roxilrlal.eLy 2 x 10 3 a-tolms
per sq.cln., -thall :in the embod-iment desc:r:i.bed wi.th re:c^t?-~
rence to F-igures 1 -to 3. The :Layer th:ickne.ss :is less,
namely ~l microns ln th:is emboclilrlent. The deep le~rel :impuri-
ty concentra-tion is pro~ided hy golcl wlLich has been in-~
1~ trocluced :i.nto the layer by ion implanta-ti.on and i.n an
'j amount approaclling t'he boroll concent;ration. In other
¦ respects the structure is similar to -that described
with rex^erence to ~igures 1 to 3. Ho~ever the operation
dif~ers at least in so far as -the transport of radiation
generated free maJority carri.ers is eff'ectt-~d th:rough
~¦ a par-t of the silicon layer in whi.ch the deep le~el trap-
`:,
ping centres provides b~T -the gold are present.
:~l 20 In this em'bodiment the compensa-ting deep
;l le~rel centres provlded by the gold provide a sensitivity
.¦ to infla=red radiation in a s~raveleng-th range extending
..j
'j from 1.1 mi.crons -to appro~i.ma-tely 3.5 microns.
-: In a second further embodimen-t wh:ich is a
further modification of the embodi.ment described ~ith
. ` l
"'~ reference -to Figures 1 to 3, the conclucti~rity types are
~ !
'~l all reversed, namely the substra-te 11 :is o:L` p-tyE~e sil:i.con.,
the laye:L 2 i.s of n-type silico:n and tlle isoLating region
8 is p--t-ype~ The n-type t.ayer ~hich in this embodiment
;' 30 ~ is o:~ l~ microns th:ickness~ is su'bs-tantially- uni.rormly
~ doped ~ritll pho-~sph-,rus in a concent:ratioll corresponcli:ng
'~` to /i x l013 atoms per cm2. 'r'he det?p :Le-vel impuri-ty con
,., ~i, .
~- ' -3-
''
P] T L, . '3 .'-' 5 '3 1
6 I 1 I '3
C(.`l~ i Oll i ~: I>~ ))' COllll)t-`Il'i~ t i ]l~ c~-f ~c l,.s ItL o
¦ ¦IC~ t`O i. ~)I1 1 )( 1111:)1 l:l.'~llll(`ll 1: il~l '; 1-(` ~!J L I) t:i ~ i ,C; l3d 1,(,) yi(~ O--
~'~ c l~ d(~ > ~-i tll d~ l) t}). :i.31 t~ 3 'I.~.~y(~:c~ ~.d
}IF~':S tl j~C.Il~ COIlCC`)ltrll;:i.011 ap~l~I`O~lCh:irI~:; tll~ 1, o:t t:tlo r~ os~)]:1.ol~u.
COnCel~tI`ai:i on, the pio-rl;:Lorl of` the l?--l;$~po l~ycr adJ.I.cen-t
l;he sll:r:f`.-~ce 3 :immQcl:i~ate:iy be:low the ::insll:L~1t-ing layer 7
having an app]-eciabl~ :l,ower defect clens:i-ty such tha-t the
phospllo:lus concentr,1-t:ioll at th:i,s are~ ncalllcly ove:r a. dl.s-
tarlce o` app:rox:;.lrlatel,y 0.1 mi CrC)31 f`:rom I;he su:r-~`ace 3, i,s
r~latively uncol11pensated. In this embocl:ime:n-t -the deep
~' levc-~l een.-t.res producecl by the cle:~ects p:rovide a sensi-
~ tivity to ini`ra-recl radiatio1l :in a waveleng,tll range ex-ten-
'I d:i,ng from 1.1 microlls to approximately 3 microns.
l 15 In this strue-ture tJl,e rel.ati~ely uncompen.--
,~ sa-tecl sur~`ace layer provides substan-tially the same ad-
vantag~es~of a higher charge han.dling capacity as provided
' by the more hig~hly doped surface 18 in the embocllment des-
eribed wi-th re:~erence to Figures 4- to 6.
. 20 '' In any of the embodiments deseribed it is
alterna-tively possible to provid.e tbe cleep level impuri-ty
I in the form oI` a plural:i-ty of diserete portions extending
i at the same leve:L in the layer rather than a continuous
~ layer portion, sa:id d:i~screte -po:r-tiorls extending locally
.~i 25 in regis-tr.ation w.i-th -the electrodes provided a-t -the major
urface of the layer.
`~ '
--3 1--
